This post is inherently misleading. It lacks serious factors that are needed to be consider and is far to short and lacks support from other studies.
One major factor missing, is the dedication of a male to his offspring.
Chimpanzee males are undedicated to the success of offspring the way that Human males are. Chimpanzee males will kill babies that they know are not theirs.
Humans behave very differently and are very likely to defend children that are very likely not their own.
Chimpanzee males also do almost nothing when it comes to raising a child, as compared to a human male which does a lot.
These references of dedicated fatherhood in human males was studied: http://news.emory.edu/stories/2013/09/esc_mens_involvement_in_child_care_study/campus.html
Smaller testicles relates to more dedicated fatherhood.
Another conflicting statement that's made is
"Among primates in which females tend to have one mate at a time, like humans and gorillas,"
Gorillas are not dedicated single male, they exhibit polyandrous behavior. There was a scientific film observation watching gorillas for years, and have found that female gorillas stray very often, which in nature gives an advantage of diversity in their offspring(sorry I forgot who the group was specifically). Gorillas exhibit Harem behavior, while chimpanzees show more alpha male like behavior.
The post also does not compare and include Bonobos, Both Chimps and Human's closest relatives. Which are far more sexual than chimps, and humans, and in addition tot hat are Female dominate in social structure, nearly the opposites of Chimps.
Bonobo males are heavily dedicated to their offspring, so much so that their the polar opposite of chimpanzee behavior.
It also does not account for 2 major aspects of humans as well.
The human penis is extremely well designed for sperm competition. https://en.wikipedia.org/wiki/Sperm_competition#Evolutionary_consequences
Yes the human penis is designed around the aspect that a female will have sex with many males. Which contradicts the statement the posted article makes.
The 2nd aspect to the contrary is that multiple male sex partner is common in human women. Most of the single partner relation ship association comes from how our culture treats relationships.
Many modern tribal women exhibit polyandrous behavior.
These are two details that directly contest that humans are monogamous.
And again refer back to the wiki page posted and
"Regarding sexual dimorphism among primates, humans falls into an intermediate group with moderate sex differences in body size but relatively large testes. This is a typical pattern of primates where several males and females live together in a group and the male faces an intermediate amount of challenges from other males compared to exclusive polygyny and monogamy but frequent sperm competition.[51]"
I'm not saying the article is completely wrong, but it is leaving out a lot of information and over simplifying to the point of being misleading.
Sorry I'm not as precise as I should be, but that article/study is very much overstating its conclusions, especially placing humans in the non poly group. Human history is vastly decorated with many forms of polyamorous behavior. So shoving us into the monogamous group as an example is inherently incorrect.